The rapid and effective repair of bone defects caused by injury, disease, wounds, surgery, etc., has long been a goal of orthopaedic surgery. Toward this end, a number of compositions and materials have been used or proposed for use in the repair of bone defects. The biological, physical, and mechanical properties of the compositions and materials are among the major factors influencing their suitability and performance in various orthopaedic applications.
Autologous cancellous bone (“ACB”) is considered the gold standard for bone grafts. ACB is osteoinductive, is non-immunogenic and, by definition, has all of the appropriate structural and functional characteristics appropriate for the particular recipient. Unfortunately, ACB is only available in a limited number of circumstances. Some individuals lack ACB of appropriate dimensions and quality for transplantation. Moreover, donor site morbidity can pose serious problems for patients and their physicians.
Much effort has been invested in the identification or development of alternative bone graft materials. Demineralized bone matrix (“DBM”) implants have been reported to be particularly useful (see, for example, U.S. Pat. Nos. 4,394,370; 4,440,750; 4,485,097; 4,678,470; and 4,743,259; Mulliken et al., Calcif. Tissue Int. 33:71, 1981; Neigel et al., Opthal. Plast. Reconstr. Surg. 12:108, 1996; Whiteman et al., J. Hand. Surg. 18B:487, 1993; Xiaobo et al., Clin. Orthop. 293:360, 1993; each of which is incorporated herein by reference). Demineralized bone matrix is typically derived from cadavers. The bone is removed aseptically and/or treated to kill any infectious agents. The bone is then particulated by milling or grinding and then the mineral component is extracted (e.g., by soaking the bone in an acidic solution). The remaining matrix is malleable and can be further processed and/or formed and shaped for implantation into a particular site in the recipient. Demineralized bone prepared in this manner contains a variety of components including proteins, glycoproteins, growth factors, and proteoglycans. Following implantation, the presence of DBM induces cellular recruitment to the site of injury. The recruited cells may eventually differentiate into bone forming cells. Such recruitment of cells leads to an increase in the rate of wound healing and, therefore, to faster recovery for the patient.
Current DBM formulations have various drawbacks. First, while the collagen-based matrix of DBM is relatively stable, the active factors within the DBM matrix are rapidly degraded. The osteogenic activity of the DBM may be significantly degraded within 24 hours after implantation, and in some instances the osteogenic activity may be inactivated within 6 hours. Therefore, the factors associated with the DBM are only available to recruit cells to the site of injury for a short time after transplantation. For much of the healing process, which may take weeks to months, the implanted material may provide little or no assistance in recruiting cells.
In addition to the active factors present within the DBM, the overall structure of the DBM implant is also believed to contribute to the bone healing capabilities of the implant.